Ceramic materials are of critical importance for a number of high temperature, high performance applications. These high temperature and high performance applications require a unique combination of properties, such as high specific strength, high temperature mechanical property retention, hardness, wear resistance, and chemical inertness, as well as low thermal and electrical conductivity.
Silicon carbide (SiC) has potential for use in high temperature structural applications because it possesses high temperature strength, oxidation and corrosion resistance, and low bulk density; however, microstructural instability at elevated temperatures often limits the use of SiC in many such applications. The morphology of SiC formed at low temperatures is cubic (beta-SiC, 3C). At higher temperatures alpha-SiC, which can have either hexagonal (alpha SiC 2H, 4H, 6H) or rhombohedral (alpha-SiC 15R, 21R) morphologies, forms. Also, at higher temperatures beta-SiC transforms into alpha-SiC. These various polytypes can undergo one or more phase transformations between 1400.degree. C. and 2200.degree. C., often resulting in brittle failure of the ceramic under structural loading.
Virkar, et.al. J. Mater. Sci., 1981, 16, 3479 have shown that the presence of 1% to 10% aluminum nitride (AlN) in hot-pressed SiC samples results in sintered parts having reduced grain size and improved microstructural uniformity. These phenomena have been attributed to the formation of solid solutions of the AlN in the SiC ceramic. In order to form an extensive solid solution, two materials must have substantially the same crystal structure, the same crystalline phase, and similar atomic or ionic sizes. The hexagonal wurtzite phase of AlN is isostructural with the 2H form of SiC and is closely lattice-matched. Consequently, AlN readily forms solid solutions with SiC and imparts phase stabilization during thermal cycling.
SiC--AlN composite and solid solution materials with a wide range of compositions have now been reported Representative examples include: Virkar, et. al. J. Mater. Sci., 1981, 16, 3479; A. Zangvil and R. Ruh in J. Am. Ceram. Soc., 1982, 66, 260-265, and in J. Am. Ceram. Soc., 1988, 71, 884-890; W-C. J. Wei and R-R. Lee in J. Mater. Sci., 1991, 26, 2930 and Ceram. Eng. Sci. Proc., 1990, 11, 1094; U.S. Pat. No. 4,141,740; and U.S. Pat. No. 3,259,509. Such SiC--AlN alloys have been shown to possess superior creep resistance, improved fracture toughness and, possibly, enhanced oxidation and corrosion resistance. In addition, the potential to vary properties such as band gap width, thermal conductivity, and thermal expansion over the range of SiC--AlN solid solution compositions makes this system attractive for many electronic applications.
The formation of dense bodies of SiC--AlN solid solutions from a mixture of SiC and AlN powder requires not only powder consolidation (sintering), but also thorough solid-state diffusion of the AlN into the SiC microstructure. The high melting points and low solid state diffusivities of both AlN and SiC, however, have limited the use of solid solution SiC--AlN ceramics. As a result, consolidated samples having representative properties have, for the most part, been prepared by pressure-assisted densification methods, e.g. hot-pressing, at relatively high temperatures of about 2200.degree. C. Such techniques are energy-inefficient and severely limit the shape-complexity of the part which can be fabricated.
A wide range of ceramic compositions, however, have now been prepared from metal organic polymer precursors. Since these polymer materials can often be formed using such conventional plastics forming techniques as coating, extrusion, and fiber spinning, a variety of intricately shaped refractory articles have been fabricated. An added advantage in the use of such polymers is their chemical reactivity. Often low temperature chemical reactions involving thermal decomposition of the polymer can be used to establish the sintered stoichiometry of the desired part in situ.
W. Rafaniello, K. Cho, and A. V. Virkar, J. Mater. Sci., 1981, 16, 3479-3488 formed SiC--AlN alloys by the carbothermal reduction of silica and alumina derived from an intimate mixture of silica, aluminum chloride, and starch. The resulting SiC--AlN powder was hot pressed, without additives, to high density.
L. D. Bentsen, D. P. H. Hasselman, and R. Ruh, J. Am. Ceram. Soc., 1983, 66, C40-C41 prepared SiC--AlN solid solutions by hot-pressing mixtures of SiC and AlN powders at 1700.degree. C. to 2200.degree. C. Specimens hot-pressed at the lower temperatures were reported to consist of composites comprising mixtures of SiC and AlN grains with only slight interdiffusion; whereas, specimens prepared at the highest temperatures were solid solutions resulting from the interdiffusion between the two components.
A. Zangvil and R. Ruh in J. Am. Ceram. Soc., 1982, 66, 260-265, and in J. Am. Ceram. Soc., 1988, 71, 884-890 describe solid solutions of SiC--AlN prepared by hot-pressing powder mixtures of silicon carbide and aluminum nitride in the temperature range 1700.degree. C. to 2300.degree. C.
W-C. J. Wei and R-R. Lee in J. Mater. Sci., 1991, 26, 2930 and Ceram. Eng. Sci. Proc., 1990, 11, 1094 describe pressureless sintered SiC/AlN composites prepared from SiC and AlN powders containing up to 4 weight % oxide sintering aids.
J. F. Janik et al., Inorg. Chem., 1987, 26, 4341-4345 reported the synthesis of the dimer {[(CH.sub.3).sub.3 Si].sub.2 AlNH.sub.2 }.sub.2 by combining [(CH.sub.3).sub.3 Si].sub.3 Al--O(C.sub.2 H.sub.5).sub.2 and ammonia in a 1:1 ratio. Upon pyrolysis in ammonia at 900.degree. C., a solid mixture of AlN/SiC forms. Variations of the reaction stoichiometry to 2:1 ammonia to [(CH.sub.3).sub.3 Si].sub.3 Al--O(C.sub.2 H.sub.5).sub.2 or addition of one equivalent of ammonia to {[(CH.sub.3).sub.3 Si].sub.2 AlNH.sub.2 }.sub.2 results in the crystalline solid compound Al{[(CH.sub.3).sub.3 Si].sub.2 Al(NH.sub.2).sub.2 }.sub.3. Pyrolysis of this compound at 930.degree. C. in ammonia gives a mixture of AlN/SiC. The report describes a discrete molecular compound as a precursor to SiC--AlN rather than an elemental silicon powder-filled polymer precursor system. The compound contains both Al--Si and Al--N bonds.
C. L. Czekja et al., in J. Am. Ceram. Soc., 1990, 73, 352-357, report the preparation of silicon carbide aluminum nitride ceramics from polymer precursors. Pyrolysis of mixtures of the carbosilane polymers [((CH.sub.3).sub.3 Si).sub.0.80 ((CH.sub.2 .dbd.CH)CH.sub.3 Si).sub.1.0 (CH.sub.3 HSi).sub.0.35 ].sub.n, or [CH.sub.3 HSiCH.sub.2 ].sub.n with the aluminum amide compound [R.sub.2 AlNH.sub.2 ].sub.3 where R.dbd.C.sub.2 H.sub.5, or i--C.sub.4 H.sub.9 yield solid solutions of 2H--SiC--AlN or SiC/AlN composites.
U.S. Pat. No. 4,687,657, T. J. Clark and R. E. Johnson, Celanese Corp., Aug. 18, 1987, formed a SiC--AlN solid solution by mixing a preceramic organosilicon polymer capable of being pyrolyzed to silicon carbide, and a preceramic organoaluminum polymer capable of being pyrolyzed to aluminum nitride, and pyrolyzing the mixed polymer composition at a temperature above 1000.degree. C. in an inert atmosphere. The silicon source employed was solely organosilicon polymers, such as poly(diorganosilanes), poly(haloorganosilanes), and poly(carbosilanes). The '657 patent discloses a method of producing SiC and AlN ceramic alloys from preceramic organic polymers by mixing a solution of a polycarbosilane with a solution of poly(N-alkylaminoalane), (HAl--NR).sub.n ' where R is an aliphatic, cycloaliphatic or aromatic hydrocarbon and n is a whole number lower than or equal to 10, and removing the solvent from the mixture of polymers and heating the solid polymeric mixture to effect the conversion of the polymers to a ceramic product.
Y. Sugahara, K-I. Sugimoto, H. Takagi, K. Kuroda, and C. Kato in J. Mater. Sci. Lett, 1988, 7, 795-797 prepared a SiC--AlN solid solution from the carbothermal reduction of a montmorillonite-polyacrylonitrile intercalation compound. The montmorillonite supplies a fixed ratio of silicon oxide, in the form of SiO.sub.2, to aluminum as the oxide Al.sub.2 O.sub.3. The intercalation compound formed from the aluminosilicate mineral, montmorillonite, and polyacrylonitrile was decarbonized under nitrogen at 650.degree. C., followed by pyrolysis at 1670.degree. C.
M. Mitomo, M. Tsutsumi, and Y. Kishi, J. Mater. Sci. Lett, 1988, 7, 1151-1153 prepared a SiC--AlN composite powder by carbothermal reduction, in nitrogen, of a mixture of SiO.sub.2, Al.sub.2 O.sub.3, and carbon at 1500.degree. C. The silica and alumina were prepared from a 1:1 ratio of Si(OC.sub.2 H.sub.5).sub.4 and Al[OCH(CH.sub.3).sub.2 ].sub.3 by hydrolysis in a solution containing carbon black.
U.S. Pat. No. 3,492,153, G. Ervin, Jr., North American Rockwell Corp., Jan. 27, 1970 described a process for preparing a high density silicon carbide-aluminum nitride refractory composition by deposition of aluminum nitride, from the vapor state, in the pores of a silicon carbide body. Preferably, the aluminum nitride was formed in situ in the pores of the silicon carbide body by heating the body at a temperature between 1400.degree. C. and 2200.degree. C. in an atmosphere containing nitrogen and vaporized aluminum. The aluminum was introduced, either as the metal or as aluminum trichloride, in the presence of ammonia. Ervin, Jr. '153, prepares a ceramic composite of AlN and SiC by taking a porous SiC preform and impregnating the preform with aluminum vapor, followed by firing.
U.S. Pat. No. 4,141,740, I. E. Cutler and P. D. Miller, University of Utah, Feb. 27, 1979, describe a solid solution ceramic and a process for preparing a solid solution ceramic including silicon carbide, aluminum oxycarbide, and aluminum nitride from intimate mixtures of fume type silica, aluminum salts and starch, or kaolinite clay and starch. The samples were dried and coked at 600.degree. C., then fired at 1500.degree. C. under nitrogen.
U.S. Pat. No. 4,904,424, R. E. Johnson, Hoechst Celanese Corp., Feb. 27, 1990, describes the formation of ceramic alloys or solid solutions formed by dispersing a powdery metal alloy or intimate mixture of two alloying metals in a precarbonaceous polymer such as polyacrylonitrile. The powdery metal alloys are selected from silicon, boron, and compounds of silicon or boron, with a second alloying metal comprising aluminum.